The Background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, typical mixers 10, 12 mix an incoming radio frequency (“RF”) signal with orthogonal local signals generated by a local oscillator (“LO”) 14. The mixers 10, 12 may receive LO signals through amplifiers 16, 18. Based on the LO signals, the mixers 10, 12 convert the RF signal into polyphase intermediate-frequency signals.
The polyphase signals may include quarter cycle segments, such as in-phase segments (I), quadrature segments (Q), inverse-phase segments (−I), and inverse-quadrature segments (−Q). Four phase signals I, Q, −I, and −Q may have the same amplitude but different adjacent phases of 0, 90, 180 and 270 degrees, respectively. The polyphase filter 20 may reject image signals of the RF input in favor of target signals.
When disturbances, such as image signals, are received in the polyphase filter 20 at a particular frequency, the polyphase filter 20 may act as a notch filter and may, for example, attenuate gain at that frequency. A notch filter blocks a narrow band of frequencies and passes all frequencies above and below the band. Notch frequency, bandwidth and depth characteristics may be controlled.
Referring now to FIGS. 2A and 2B, a passive polyphase filter may be composed of a four phase resistance and capacitance (RC) network 30 or impedance network with inputs (Vlin+, VQin+, Vlin−, VQin−) and outputs (Vlout+, VQout+, Vlout−, VQout−) disposed in the four relative phases, I, Q, −I, −Q. The four phase RC network 30 includes a resistance R in each of the four phases, which is connected between the input and output terminals 32, 34 of the associated phase. The input of each phase is connected to the output of an adjacent leading phase via a capacitance C. The RC network 30 may be represented by an asymmetrical frequency domain transfer function:
                                          V            Iout                                V            Iin                          =                              1            +                          ω              ⁢                                                          ⁢              RC                                            1            +                          jω              ⁢                                                          ⁢              RC                                                          (                  Equation          ⁢                                          ⁢          1                )            such that when ω (angular frequency) equals 1/RC, Vlout equals Vlin(1−j), and when ω equals −1/RC, Vlout equals zero. Controlling notch characteristics of the RC network 30 may be realized by replacing conventional resistances with relatively large switched resistances arrays 35, which may cause distortion. Further, the network transfer function generally does not exhibit substantial low-pass filter functions.
Referring now to FIG. 3, a frequency response of the passive polyphase filter of FIG. 2A is illustrated. In FIG. 3, graph 42 of the positive frequency response represents the characteristics when a target signal is input. Graph 44 of the negative frequency response represents the characteristics when an image signal (image rejection characteristic) is input, including a notch characteristic 46 at 1/(2πRC). The polyphase filter may reject part of the negative frequencies while passing the positive frequencies. Further, the polyphase filter may fully reject an input sequence at the RC frequency.
Referring now to FIGS. 4A and 4B, several stages of polyphase filters 50, 52, 54 may be cascaded for image rejection across a wide band. Therefore, the circuits 56, 58 before and after the polyphase filters 50, 52, 54 are usually amplifiers. However, these amplifiers may add system noise. For multistage polyphase filtering, signal strength loss may accumulate, which necessitates interstage amplifiers 60.
In FIG. 5, a typical receiver 70 may include mixers 82, with both LO and RF input circuits 90, 92, respectively applied in quadrature. The outputs of the mixers 82 are combined following processing in amplifiers 93 and polyphase filter 94.